The CDC reports at https://www.cdc.gov/coronavirus/2019-ncov/science/science-br... that "in-person learning in schools has not been associated with substantial community transmission"

The low symptom rate corresponds to a low transmission rate. The worse your case of COVID, the more likely you will transmit it to others in the same situation. This is the obvious mechanism of the fact that kids, who very seldom have bad cases of COVID and so frequently do not show symptoms, are not usually major spreaders of the virus.

This all is one of the really unusual things about COVID compared to other communicable diseases.